Even when fully charged, a cell phone battery continues to draw electricity for as long as it's connected to the electricity supply. Leaving your cell phone plugged in does waste some power, though it's a relatively small amount. If you leave your phone plugged in for very long periods, overheating can occur.
Vampire drain is the unnecessary power drain by devices left on standby or fully-charged battery devices left connected to the electricity supply. In the case of cell phone batteries, this drain is small. The Lawrence Berkeley National Laboratory estimates that a cell phone draws roughly 3.68 watts of power while charging and 2.24 watts once charged. Overcharging for eight hours every night for a year, your phone might waste 6.5 kWh (kilowatt hours). For comparison, the energy consumption for an average American is over 4,000 kWh per year.
It is possible for a cell phone or cell phone charger to overheat if left connected to the power supply for long periods. In 2005, a house fire in Dutchess County, New York, was blamed on a cell phone charger. Authorities surmised that the charger had overheated and short-circuited, causing the fire. Significant overheating may reduce the life span of some cell phone batteries. An old or damaged battery is more likely to overheat through overcharging than a new one.
The "Memory Effect"
This is an effect seen in nickel cadmium (NiCd) and nickel-metal hydride (NiMH) cell phone batteries. It occurs when the battery is not discharged completely before being recharged. If you do this on a regular basis, your battery can develop a "memory." For example, if your battery had an hour of charge left and you recharged it, the battery could "forget" that hour in the future, reducing its performance. Proper battery care prevents this.
Lithium-ion cell phone batteries must be properly conditioned before you use them. This means fully charging and discharging the battery three times before you begin using the battery normally. It is actually recommended that you overcharge the battery slightly during this conditioning process, for example, by leaving it plugged in overnight, as this ensures your battery is fully charged. Lithium-ion batteries do not suffer from the memory effect.
Wireless charging uses a charging mat. You place your cell phone or other battery-powered device on the mat where it is charged by induction. Your cell phone needs to be monitored more closely when doing wireless charging, as the mat can become hot, although it will cool down once the battery is fully charged.